Ink set and duplex printing method

ABSTRACT

According to the invention, there is provided an ink set which contains at least one ink selected from a group of a black pigment ink, a self-dispersing color pigment ink, and a resin-dispersion color pigment ink. The moisture quantity of the black pigment ink is from 68 mass % to 75 mass %. The moisture quantity of the self-dispersing color pigment ink is from 62 mass % to 72 mass %. The moisture quantity of the resin-dispersion color pigment ink is from 68 mass % to 75 mass %.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.15/168,718, filed May 31, 2016, which claims priority to Japanese PatentApplication No. 2015-128360, filed Jun. 26, 2015, both of which arehereby expressly incorporated by reference herein in their entireties.

BACKGROUND 1. Technical Field

The present invention relates to an ink set and a duplex printingmethod.

2. Related Art

Speeding-up of an ink jet recording method using an aqueous ink has beendeveloped. Among technologies for the speeding-up, a line type ink jetprinter which uses a line head as a recording head has a printing speedof about 100 pages/minute and can perform high density printing.

However, the line type ink jet printer easily performs high-speedsimplex printing, but speeding-up of duplex printing is difficult mainlydue to two problems. A first problem is that moisture contained in anink causes sheet deformation such as curling or cockling to occur on asheet after simplex printing, clogging of a sheet transported on a sheettransporting path (occurrence of paper jam) causes sheet transporting tobe difficult. A second problem is that infiltration or evaporation of anaqueous ink to a sheet takes time, and thus the ink may be transferredto a sheet transporting mechanism such as a sheet feeding roller. Withsuch problems, in the line type ink jet printer using an aqueous ink,the speed of duplex printing is significantly slow, that is, only 2pieces/minute to 3 pieces/minute, and thus realization of a duplexprinting speed such as the speed of a laser printer is difficult.

Thus, for example, JP-A-2012-183798 discloses a technology as follows.That is, in a case where a picture, a drawing, and the like are printedon plain paper at a high speed by using the ink jet recording apparatus,a large amount of an ink adheres to the plain paper, and thus sufficientsuppression of the occurrence of the curling after recording is notpossible. In order to solve such problem, a heating section is providedin a liquid ejecting apparatus and a first surface of a sheet is dried.Thus, a predetermined difference is provided between the moisturequantity of the first surface of the sheet and the moisture quantity ofthe second surface of the sheet, and an aqueous liquid is ejected and isadhered to the dried first surface of the sheet.

For example, JP-A-2006-256289 discloses a technology as follows. Thatis, in order to provide an ink jet printing device in which duplexprinting is performed without a reversing mechanism of a printing mediumor a transfer medium, ink jet type recording heads are disposed overboth surfaces of a printing medium which is transported vertically to asimple downward path from the top of the device, and duplex printing isperformed by using a quick-drying aqueous ink which has high viscosity.

However, in a case where a drying process for high-speed printing isprovided, a waiting period until infiltration is completed is provided,or printing is simultaneously performed on both surfaces, mechanicalefforts are required. In a case where an aqueous ink having a largemoisture quantity is used, suppression of cockling after printing, orsuppression of the occurrence of permanent curling of the ejected sheetis difficult and speeding-up of the duplex printing by improving sheettransporting properties is difficult. In a case where a high-viscosityink having a small moisture quantity is used, the ink is rapidly dried,but infiltration to a sheet is slow. Thus, speeding-up of the duplexprinting is difficult.

In this manner, it is difficult that infiltration of an ink to a sheetor drying of the ink is completed in a second unit. Particularly, ifprinting is performed on the back surface just after printing isperformed on the front surface for the purpose of high-speed duplexprinting, back surface printing is started in a state where theinfiltration of the ink to the front surface is not completed. Thus, aproblem in that the ink is transferred to the sheet transportingmechanism such as a sheet feeding roller, the ink is transferred to thesheet again, and thus the sheet is stained with the ink occurs.

SUMMARY

An advantage of some aspects of the invention is to provide an ink setand a duplex printing method in which transfer of an ink to a sheettransporting member is suppressed, sheet transporting properties areimproved, and thus speeding-up of duplex printing is caused to bepossible.

The invention can be realized in the following aspects or applicationexamples.

APPLICATION EXAMPLE 1

According to an aspect of the invention, there is provided an ink setwhich contains at least one ink selected from a group of a black pigmentink, a self-dispersing color pigment ink, and a resin-dispersion colorpigment ink. A moisture quantity of the black pigment ink is from 68mass % to 75 mass %. A moisture quantity of the self-dispersing colorpigment ink is from 62 mass % to 72 mass %. A moisture quantity of theresin-dispersion color pigment ink is from 68 mass % to 75 mass %.

According to Application Example 1, the moisture quantities of the inksconstituting the ink set are adjusted, and thus it is possible toprovide an ink set which can suppress occurrence of transfer of the inkand deformation of a sheet, and can cause a speed of duplex printing tobe increased.

APPLICATION EXAMPLE 2

In the ink set according to Application Example 1, each of the blackpigment ink, the self-dispersing color pigment ink, and theresin-dispersion color pigment ink may contain three or more types ofacetylene glycol-based surfactants selected from a group of acetyleneglycol and an ethylene oxide adduct of acetylene glycol, the totalcontent of the acetylene glycol-based surfactants may be from 0.1 mass %to 3 mass % with respect to the total mass of each of the black pigmentink, the self-dispersing color pigment ink, and the resin-dispersioncolor pigment ink.

APPLICATION EXAMPLE 3

In the ink set according to

APPLICATION EXAMPLE 1 or APPLICATION EXAMPLE 2, the ink set may be usedfor a duplex-printing ink jet recording apparatus. APPLICATION EXAMPLE 4

In the ink set according to any one example of

APPLICATION EXAMPLE 1 to APPLICATION EXAMPLE 3, the total viscosity ofthe black pigment ink, the self-dispersing color pigment ink, and theresin-dispersion color pigment ink may be from 2.0 mm²/s to 4.0 mm²/s.APPLICATION EXAMPLE 5

In the ink set according to any one example of

APPLICATION EXAMPLE 1 to APPLICATION EXAMPLE 4, each of the blackpigment ink, the self-dispersing color pigment ink, and theresin-dispersion color pigment ink may further contain alkylene glycolmonoalkyl ether. APPLICATION EXAMPLE 6

In the ink set according to any one example of Application Example 1 to

APPLICATION EXAMPLE 5, each of the black pigment ink, theself-dispersing color pigment ink, and the resin-dispersion colorpigment ink may further contain 1,2-alkane diol. APPLICATION EXAMPLE 7

In the ink set according to any one example of Application Example 1 toApplication Example 6, each of the black pigment ink, theself-dispersing color pigment ink, and the resin-dispersion colorpigment ink may further contain trimethylglycine.

APPLICATION EXAMPLE 8

According to another aspect of the invention, there is provided an inkset. A moisture quantity of a black pigment ink is from 68 mass % to 75mass %. A moisture quantity of a self-dispersing color pigment ink isfrom 62 mass % to 72 mass %. A moisture quantity of a resin-dispersioncolor pigment ink is from 68 mass % to 75 mass %.

APPLICATION EXAMPLE 9

In the ink set according to any one example of Application Example 1 toApplication Example 8, the black pigment ink may be formed from aself-dispersing pigment.

APPLICATION EXAMPLE 10

According to still another aspect of the invention, there is provided aduplex printing method of performing duplex printing by using an ink jetrecording apparatus. The ink jet recording apparatus includes an ink setand a sheet transporting mechanism. The ink set contains at least oneselected from a group of a black pigment ink, a self-dispersing colorpigment ink, and a resin-dispersion color pigment ink. A moisturequantity of the black pigment ink is from 68 mass % to 75 mass %. Amoisture quantity of the self-dispersing color pigment ink is from 62mass % to 72 mass %. A moisture quantity of the resin-dispersion colorpigment ink is from 68 mass % to 75 mass %. In the sheet transportingmechanism, a period from when printing on a front surface is started,until printing on a back surface is started is within three seconds.

APPLICATION EXAMPLE 11

In the duplex printing method according to Application Example 10, aduplex printing speed (page/minute) in printing a solid image on theentirety of a surface of plain paper may be from 30% of a simplexprinting speed, to 100% thereof.

APPLICATION EXAMPLE 12

In the duplex printing method according to Application Example 10 orApplication Example 11, the black pigment ink may be formed from aself-dispersing pigment.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described with reference to the accompanyingdrawings, wherein like numbers reference like elements.

FIGURE is a schematic diagram illustrating a side sectional structure ofan ink jet recording apparatus used in an embodiment.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

Hereinafter, preferred embodiments will be described. The embodimentswhich will be described below are used for describing an example of theinvention. The invention is not limited to the following embodiments andmay include various modification examples conducted in a range withoutchanging from the gist of the invention.

1. INK SET

An ink set according to an embodiment of the invention contains at leastone selected from a group of a black pigment ink, a self-dispersingcolor pigment ink, and a resin-dispersion color pigment ink. A moisturequantity of the black pigment ink is from 68 mass % to 75 mass %. Amoisture quantity of the self-dispersing color pigment ink is from 62mass % to 72 mass %. A moisture quantity of the resin-dispersion colorpigment ink is from 68 mass % to 75 mass %. The ink set is mounted, forexample, in an ink jet recording apparatus which includes a sheettransporting mechanism (which will be described later) in which a periodfrom when printing on a front surface is started, until printing on aback surface is started is within three seconds. The ink set is used forperforming duplex printing by using a line type ink jet recording methodwith a line type recording head.

An ink constituting the ink set according to the embodiment, componentscontained in the ink, and components which may be contained will bedescribed below. The ink set according to the embodiment contains atleast one selected from a group of the black pigment ink, theself-dispersing color pigment ink, and the resin-dispersion colorpigment ink which respectively have the above-described moisturequantities. As long as an ink set contains at least one of the inks, theink set is assumed to constitute the ink set according to theembodiment.

1.1. Ink

Each of the inks constituting the ink set according to the embodiment(black pigment ink, self-dispersing color pigment ink, andresin-dispersion color pigment ink which respectively have theabove-described moisture quantities) may contain a pigment as acolorant. The pigment may use any of an inorganic pigment and an organicpigment.

1.1.1. Black Pigment Ink

The black pigment ink constituting the ink set according to theembodiment contains a black pigment and water. The moisture quantity ofthe black pigment ink is from 68 mass % to 75 mass %.

As the pigment used in the black pigment ink, carbon black (C.I. PigmentBlack 7) is preferable. Examples of the carbon black include furnaceblack, lamp black, acetylene black, and channel black.

Examples of commercial products of carbon black include carbon blackNo.2300, No.900, MCF88, No.33, No.40, No.45, No.52, MA7, MA8, MA100,No.2200B, and the like (all the above are merchandise names of productsmanufactured by Mitsubishi Chemical Corporation); Raven H20, 5750, 5250,5000, 3500, 1255, 700, and the like (all the above are merchandise namesof products manufactured by Carbon Columbia Corporation); Regal 400R,330R, 660R, Mogul L, Monarch 700, 800, 880, 900, 1000, 1100, 1300, 1400,and the like (all the above are merchandise names of productsmanufactured by Cabot Japan K.K.); Color Black FW1, FW2, FW2V, FW18,FW200, 5150, 5160, 5170, Printex 35, U, V, 140U, Special Black 6, 5, 4A,4, and the like (all the above are merchandise names of productsmanufactured by Degussa Corporation); and BONJET BLACK M-800(merchandise name of a product manufactured by Orient ChemicalIndustries Co., Ltd.).

The carbon black may be singly used or be used in combination of two ormore types.

In order to obtain excellent concealing property and colorreproducibility, the content of the carbon black is preferably from 1mass % to 15 mass % with respect to the total mass (100 mass %) of theink.

The average particle diameter of the carbon black is preferably from 50nm to 500 nm. If the average particle diameter is equal to or more than50 nm, a color-forming property is good. Thus, the carbon black iseasily used as the ink. If the average particle diameter is equal to orless than 500 nm, the carbon black is easily used in an ink jet method.In order to obtain excellent storage stability, ejecting stability, andsettleability of the ink, the average particle diameter is morepreferably from 50 nm to 300 nm, and further preferably from 50 nm to200 nm.

Here, the term of the “average particle diameter” in this specificationrefers to a sphere conversion 50% average particle diameter (d50)obtained by a light scattering method. The term of the “sphereconversion 50% average particle diameter (d50) by the light scatteringmethod” corresponds to a value obtained in the following manner.Particles in a dispersion medium are irradiated with light and detectorswhich are respectively disposed in the front, the side and the rear ofthe dispersion medium measure diffractive scattering light which isgenerated by the irradiation. A particle which has originally anindeterminate form is assumed to be spherical by using the measuredvalue. The particle is converted into a sphere having the same volume ofthe particle, and the total volume of a particle group is set to be 100%so as to obtain an accumulation curve. At this time, a point at whichthe accumulated value is 50% is set as the 50% average particle diameter(d50).

The black pigment ink has a moisture quantity which is from 68 mass % to75 mass % with respect to the total mass (100 mass %) of the ink. In acase where the moisture quantity is in this range, it is possible toprovide an ink set which can suppress the occurrence of transfer of theink and deformation of a sheet, and can cause a speed of duplex printingto be increased. In a case where the moisture quantity is less than 68mass o, the viscosity of the ink is too high, and thus infiltration to asheet becomes slow and an amount of the ink transferred to the sheettransporting mechanism is increased. In the moisture quantity is morethan 75 mass o, water causes a sheet to be largely deformed, cocklingoccurs or permanent curling is generated, and sheet transporting isdifficult. In order to suppress the occurrence of deformation of a sheetdue to water, the moisture quantity is more preferably from 70 mass % to73 mass %.

As the water, pure water or ultra-pure water such as ion exchange water,ultrafiltration water, reverse osmotic water, and distilled water, thatis, water in which ionic impurities are removed as much as possible ispreferable. If water sterilized by ultraviolet ray radiation, additionof hydrogen peroxide, or the like is used, generation of mold orbacteria in a case where the ink is stored for a long term can beprevented. Thus, such water is desired.

As will be described below, the black pigment ink may be formed from aself-dispersing pigment which has a hydrophilic group on the surface ofthe pigment.

1.1.2. Self-Dispersing Color Pigment Ink

The self-dispersing color pigment ink constituting the ink set accordingto the embodiment contains water and a self-dispersing color pigment. Inthe self-dispersing color pigment, a hydrophilic functional group ischemically or physically provided on the surface of a pigment particle,in order to enable the pigment to be stably dispersed and maintained inwater is used. The moisture quantity of the self-dispersing colorpigment ink is from 62 mass % to 72 mass %.

Here, the self-dispersing pigment refers to a pigment which can bedispersed or dissolved in an aqueous medium without a dispersant. Here,the term of “dispersing or dissolving in an aqueous medium without adispersant” means a state of stably being present in the aqueous mediumby the hydrophilic group on the surface of the pigment, even when thedispersant for dispersing the pigment is not used. Thus, an ink in whichfoaming hardly occurs due to the dispersant, and the ejecting stabilityis excellent is easily produced. Since a significant increase of theviscosity due to the dispersant is suppressed, the pigment is easilyhandled, for example, the pigment may be contained more or the printingdensity may be sufficiently increased.

The hydrophilic group is preferably one or more hydrophilic groupsselected from a group of —OM, —COOM, —CO—, —SO₃M, —SO₂M, —SO₂NH₂,—RSO₂M, —PO₃HM, —PO₃M₂, —SO₂NHCOR, —NH₃. and —NR₃ (in the aboveformulas, M indicates a hydrogen atom, alkali metal, ammonium, a phenylgroup which may have a substituent, or organic ammonium, and R indicatesan alkyl group having 1 to 12 carbon atoms or a naphthyl group which mayhave a substituent).

The self-dispersing pigment is manufactured, for example, in such amanner that the following color pigment is subjected to a physicaltreatment or a chemical treatment so as to bond (graft) the hydrophilicgroup to the surface of the pigment. As the physical treatment, forexample, a vacuum plasma treatment may be exemplified. As the chemicaltreatment, for example, a wet oxidation method of performing oxidizationwith an oxidant in water, a method in which a carboxyl group is bondedthrough a phenyl group by bonding p-aminobenzoic acid to the surface ofthe pigment, or the like may be exemplified.

A color pigment used in the self-dispersing color pigment ink may beappropriately exemplified in accordance with the type (color) of an inkto be obtained.

Examples of a pigment for a yellow ink include C.I. Pigment Yellow 1, 2,3, 12, 14, 16, 17, 73, 74, 75, 83, 93, 95, 97, 98, 109, 110, 114, 128,129, 138, 139, 147, 150, 151, 154, 155, 180, and 185. Among thepigments, one or more types selected from a group of C.I. Pigment Yellow74, 110, 128, and 129 are preferably used.

Examples of a pigment for a magenta ink include C.I. Pigment Red 5, 7,12, 48(Ca), 48(Mn), 57(Ca), 57:1, 112, 122, 123, 168, 184, 202, and 209,and C.I. Pigment Violet 19. Among the pigments, one or more typesselected from a group of C.I. Pigment Red 122, 202, and 209, and C.I.Pigment Violet 19 are preferably used. In addition, a solid solution ofthe above substances may be used.

Examples of a pigment for a cyan ink include C.I. Pigment Blue 1, 2, 3,15:2, 15:3, 15:4, 15:34, 16, 22, and 60, C.I. Vat Blue 4 and 60. Amongthe pigments, at least one of C.I. Pigment Blue 15:3 and C.I. PigmentBlue 15:4 is preferably used, and C.I. Pigment Blue 15:3 is morepreferably used.

The color pigment may be singly used or be used in combination of two ormore types.

The content of the color pigment is not particularly limited as long asthe content may be adjusted to be an appropriate content (pigmentconcentration) when the ink is prepared. For example, a light ink inwhich a pigment concentration is from 1 mass % to 3 mass % is used, andthus an image in which granularity is suppressed may be obtained. Anormal ink in which the pigment concentration is from 4 mass % to 12mass % is used, and thus an image having an excellent color-formingproperty may be obtained.

The average particle diameter of the color pigment is preferably from 50nm to 500 nm. If the average particle diameter is equal to or more than50 nm, the color-forming property becomes better, and thus the colorpigment is easily used as an ink. If the average particle diameter isequal to or less than 500 nm, the color pigment is easily used in theink jet method. In order to obtain excellent storage stability, ejectingstability, and settleability of the ink, the average particle diameteris more preferably from 50 nm to 300 nm, and further preferably from 50nm to 200 nm.

The self-dispersing color pigment ink has a moisture quantity which isfrom 62 mass % to 72 mass % with respect to the total mass (100 mass %)of the ink. In a case where the moisture quantity is in this range, itis possible to provide an ink set which can suppress the occurrence oftransfer of the ink and deformation of a sheet, and can cause a speed ofduplex printing to be increased. In a case where the moisture quantityis less than 62 mass %, the viscosity of the ink is too high, and thusinfiltration to a sheet becomes slow and an amount of the inktransferred to the sheet transporting mechanism is increased. In themoisture quantity is more than 72 mass %, water causes a sheet to belargely deformed, cockling occurs or permanent curling is generated, andsheet transporting is difficult. In order to suppress the occurrence oftransfer of an ink to a double-side roller and the occurrence ofdeformation of a sheet due to water, the moisture quantity is morepreferably from 66 mass % to 70 mass %.

Similarly to the black pigment ink, as the water, pure water orultra-pure water such as ion exchange water, ultrafiltration water,reverse osmotic water, and distilled water, that is, water in whichionic impurities are removed as much as possible is preferable. If watersterilized by ultraviolet ray radiation, addition of hydrogen peroxide,or the like is used, generation of mold or bacteria in a case where theink is stored for a long term can be prevented. Thus, such water isdesired.

1.1.3. Resin-Dispersion Color Pigment Ink

The resin-dispersion color pigment ink constituting the ink setaccording to the embodiment contains water and a resin-dispersionpigment. The resin-dispersion pigment may be dispersed by resindispersion in order to enable the pigment to be stably dispersed in thewater. The moisture quantity of the resin-dispersion pigment ink is from68 mass % to 75 mass %.

A case where a hydrophilic group is attached to the surface of a yellowpigment so as to be used as a self-dispersing yellow pigment ink may betechnically difficult. Thus, as the yellow ink, a resin-dispersionyellow pigment ink is preferably used. As a pigment included in theresin-dispersion yellow pigment ink, an azo-based pigment, a condensedring-based pigment, and the like may be exemplified in addition to apigment such as Pigment Yellow disclosed in the Color Index. An organicpigment such as Yellow No. 4, No. 5, No. 205, and No. 401 isexemplified. A specific example of the yellow pigment includes C.I.Pigment Yellow 1, 3, 12, 13, 14, 17, 24, 34, 35, 37, 42, 53, 55, 74, 81,83, 95, 97, 98, 100, 101, 104, 108, 109, 110, 117, 120, 128, 138, 150,153, 155, 174, 180, and 198. Among the pigments, it is preferable thatan organic pigment included in the yellow ink contains at least oneselected from a group of C.I. Pigment Yellow 74, 109, 110, 128, 138,147, 150, 155, 180, and 188.

The pigment may be singly used or be used in combination of two or moretypes.

The pigment is preferably contained so as to be equal to or more than 6mass %. In a case where the pigment concentration is equal to or morethan 6 mass %, a record is excellent in color-forming property.

The average particle diameter of the pigment is preferably from 50 nm to500 nm. If the average particle diameter is equal to or more than 50 nm,the color-forming property becomes better, and thus the color pigment iseasily used as an ink. If the average particle diameter is equal to orless than 500 nm, the color pigment is easily used in the ink jetmethod. In order to obtain excellent storage stability, ejectingstability, and settleability of the ink, the average particle diameteris more preferably from 50 nm to 300 nm, and further preferably from 50nm to 200 nm.

A resin (polymer) used in the resin-dispersion pigment is not limited tothe following descriptions. However, for example, Tg of dispersionpolymer used in dispersion of the pigment is preferably equal to orlower than 55° C., and more preferably equal to or lower than 50° C. IfTg is equal to or lower than 55° C., fixability can be improved. Polymerhaving a weight average molecular weight (in terms of styrene), which isobtained by gel permeation chromatography (GPC) and is from 10,000 to200,000, is preferably used. Thus, the storage stability as the pigmentink is further improved.

Here, the weight average molecular weight in this specification ismeasured as a weight average molecular weight in terms of stylene, byusing the gel permeation chromatography (GPC) of L7100 system(manufactured by Hitachi Ltd.).

If a polymer in which a component of 70 mass % or more is a copolymer of(meta)acrylate and (meta)acrylic acid is used as the above polymer,fixability and glossiness is further improved. A polymer obtained bypolymerizing a monomer component in which at least one of alkyl(meta)acrylate having carbon atoms of 1 to 24 and cyclic alkyl(meta)acrylate having carbon atoms of 3 to 24 is equal to or more than70 mass % is preferable. A specific example of such a polymer includesmethyl (meta)acrylate, ethyl (meta)acrylate, propyl (metal)acrylate,n-butyl (meta)acrylate, isobutyl (meta)acrylate, pentyl (meta)acrylate,hexyl (meta)acrylate, 2-ethylhexyl (meta)acrylate, octyl (meta)acrylate,nonyl (meta)acrylate, decyl (meta)acrylate, t-butyl cyclohexyl(meta)acrylate, lauryl (meta)acrylate, isobornyl (meta)acrylate, cetyl(meta)acrylate, stearyl (meta)acrylate, isostearyl (meta)acrylate,tetramethylpiperidyl (meta)acrylate, dicyclopentanyl (meta)acrylate,dicyclopentenyl (meta)acrylate, dicyclopentenyloxy (meta)acrylate, andbehenyl (meta)acrylate. As other additive components, hydroxy(meta)acrylate which has a hydroxyl group, urethane (meta)acrylate, andepoxy (meta)acrylate may be also used. Examples of the hydroxy(meta)acrylate include hydroxyethyl (meta)acrylate, hydroxypropyl(meta)acrylate, and diethylene glycol (meta)acrylate.

For further improving fixability, glossiness, and color producibility,among the resin-dispersion pigments, a pigment (microcapsule typepigment) coated with polymer is appropriately used.

The pigment coated with polymer is obtained by using a phase reversalemulsification method or a sedimentation deposition method. In the phasereversal emulsification method, firstly, the polymer is dissolved in anorganic solvent such as methanol, ethanol, isopropanol, n-butanol,acetone, methyl ethyl ketone, and dibutyl ether. The pigment is added tothe obtained solution and then a neutralizing agent and water are added.A kneading and dispersing treatment is performed and thus anoil-in-water type dispersion matter is prepared. The organic solvent isremoved from the obtained dispersion matter, and thus the pigment coatedwith polymer may be obtained as a water dispersion matter. The kneadingand dispersing treatment may be performed, for example, by using a ballmill, a roll mill, a bead mill, a high-pressure homogenizer, ahigh-speed stirring type disperser, and the like.

As the neutralizing agent, trivalent amine such as ethylamine andtrimethylamine, lithium hydroxide, sodium hydroxide, potassiumhydroxide, ammonia, and the like are preferably used. pH of the obtainedwater dispersion matter is preferably from 6 to 10.

As the polymer for coating the pigment, a substance in which the weightaverage molecular weight measured by the GPC is about from 10,000 to150,000 is preferably used because the substance causes the pigment tobe stably dispersed.

The content ratio of the pigment dispersion resin may be appropriatelyselected depending on a pigment to be dispersed. However, the contentratio of the pigment dispersion resin is preferably from 5 parts by massto 200 parts by mass, and more preferably from 30 parts by mass to 120parts by mass, with respect to 100 parts by mass as the content of thepigment in an ink.

The resin-dispersion color pigment ink has a moisture quantity which isfrom 68 mass % to 75 mass % with respect to the total mass (100 mass %)of the ink. In a case where the moisture quantity is in this range, itis possible to provide an ink set which can suppress the occurrence oftransfer of the ink and deformation of a sheet, and can cause a speed ofduplex printing to be increased. In a case where the moisture quantityis less than 68 mass %, the viscosity of the ink is too high, and thusinfiltration to a sheet becomes slow and an amount of the inktransferred to the sheet transporting mechanism is increased. In themoisture quantity is more than 75 mass %, water causes a sheet to belargely deformed, cockling occurs or permanent curling is generated, andsheet transporting is difficult. In order to suppress the occurrence ofdeformation of a sheet, the moisture quantity is more preferably from 70mass % to 73 mass %.

As the water, pure water or ultra-pure water such as ion exchange water,ultrafiltration water, reverse osmotic water, and distilled water, thatis, water in which ionic impurities are removed as much as possible ispreferable. If water sterilized by ultraviolet ray radiation, additionof hydrogen peroxide, or the like is used, generation of mold orbacteria in a case where the ink is stored for a long term can beprevented. Thus, such water is desired.

1.2. Acetylene Glycol-Based Surfactant

Each of the inks constituting the ink set according to the embodimentmay contain three or more types of acetylene glycol-based surfactantsselected from a group of acetylene glycols and ethylene oxide adducts ofacetylene glycol. In this case, the total content of the acetyleneglycol-based surfactants is from 0.1 mass % to 3 mass % with respect tothe total mass of each of the inks constituting the ink set.

In a case where each of the inks constituting the ink set according tothe embodiment contains three or more types of acetylene glycol-basedsurfactants which have the above-described specific structure, so as tocause the content of the acetylene glycol-based surfactants to be from0.1 mass % to 3 mass % with respect to the total mass of each of theinks, the ink is infiltrated to a sheet rapidly. Thus, an effect ofsuppressing the occurrence of transfer of the ink to the sheettransporting mechanism is obtained.

In the following descriptions, in this specification, a case of beingreferred to as “three or more types of acetylene glycol-basedsurfactants” indicates three or more types acetylene glycol-basedsurfactants which are selected from a group formed by the followingformula (1) and the following formula (2).

In the formula (1), each of R¹ and R² independently indicates an alkylgroup having carbon atoms of 1 to 5. The alkyl group having carbon atomsof 1 to 5 may have a straight-chain structure or have a branched-chainstructure. A specific example of such an alkyl group includes a methylgroup, an ethyl group, an n-propyl group, an isopropyl group, an n-butylgroup, an isobutyl group, a sec-butyl group, a tert-butyl group, apentyl group, an isopentyl group, a neopentyl group, and the like. R¹and R² may be the same as each other or be different from each other.

In the formula (2), each of R³ and R⁴ independently indicates an alkylgroup having carbon atoms of 1 to 5. Each of m and n indicates apositive number of 0.5 to 25, and (m+n) is from 1 to 40. The alkyl grouphaving carbon atoms of 1 to 5 may have a straight-chain structure orhave a branched-chain structure. A specific example of such an alkylgroup includes a methyl group, an ethyl group, an n-propyl group, anisopropyl group, an n-butyl group, an isobutyl group, a sec-butyl group,a tert-butyl group, a pentyl group, an isopentyl group, a neopentylgroup, and the like. R³ and R⁴ may be the same as each other or bedifferent from each other. Each of m and n indicates the number of molesof added ethylene oxide. (m+n) is required to be from 1 to 40. However,(m+n) is preferably from 10 to 30, and more preferably from 15 to 25.

It is considered that the reason of a mechanism in which an ink whichcontains three or more types of acetylene glycol-based surfactantshaving the above-described specific structure is used, and thus aneffect of rapidly infiltrating the ink to a sheet can be exhibited is asfollows.

If a plurality of surfactants which have a structure similar to eachother is provided in the ink, arrangement of each of the surfactants isdisturbed, forming a large micelle formed from one surfactant isdifficult, and each of the surfactants is present in a form of a minutecluster. As a result, a moving speed of the ink on a surface of arecording medium is increased, and wettability and an infiltrationproperty to the recording medium are improved. Thus, it is possible torealize improvement of image quality and speeding-up in recording. Sucha phenomenon occurs further significantly by causing the three or moretypes of acetylene glycol-based surfactants which have a structuresimilar to each other, to be provided.

It is known that an infiltration speed of the ink to a recording mediumis proportional to a value calculated from an expression represented byγ·cosθ (here, γ indicates surface tension of the ink and θ indicates acontact angle of the ink with the recording medium). Here, afluorine-based surfactant or a silicone-based surfactant which has beenwidely used in an ink for ink jet recording has a small contact angle θ,but also has small surface tension γ. Thus, improvement of theinfiltration speed of the ink to the recording medium is not expected.On the contrary, the acetylene glycol-based surfactant has a smallcontact angle, but has high surface tension γ. Thus, it is possible torealize improvement of the infiltration speed of the ink, improvement ofimage quality and fixability, and speeding-up in recording. Such anaction is further significantly exhibited by causing the three or moretypes of acetylene glycol-based surfactants which have a structuresimilar to each other, to be provided.

A status of image quality in this specification may be determined basedon a state such as aggregation of ink droplets, color unevenness,bleeding, contamination of an image due to transfer of the ink to aroller, and the like. Here, the aggregation of ink droplets means aphenomenon occurring by that an ink droplet attached to the recordingmedium flows before the recording medium is spread wet, and thus theflowing ink droplet is combined to another ink droplet. If suchaggregation of ink droplets occurs, streak unevenness, color unevenness,or the like may occur. The streak unevenness represents a state where aportion of which the surface of the recording medium is not coated withink droplets remains so as to have a stripe shape. The color unevennessmay occur in an image, by a local irregularity of density. The bleedingrefers to a state where an outline of an image recorded with a singlecolor is blurred and thus the image is not clear, or one adjacent coloris blurred to another color side in a border between different colorsand thus the colors are mixed to each other, and the image is not clear.

A specific example of acetylene glycol represented by the formula (1)includes 2,5,8,11-tetramethyl-6-dodecyne-5,8 -diol,5,8-dimethyl-6-dodecyne-5,8-diol, 2,4,7,9-tetramethyl-5-decyne-4,7-diol,4,7-dimethyl-5-decyne-4,7-diol, 2,3,6,7-tetramethyl-4-octyne-3,6-diol,3,6-dimethyl-4-octyne-3,6-diol, 3,6-diethyl-4-octyne-3,6-diol, and2,5-dimethyl-3-hexyne-2,5-diol. As the acetylene glycol represented bythe formula (1), commercial product may be used. For example, Surfynol82, 104, and DF-110 (all the above are merchandise name, manufactured byNissin Chemical Co., Ltd.) may be included.

A specific example of an ethylene oxide adduct of acetylene glycolrepresented by the formula (2) includes ethylene oxide adducts ofcompounds exemplified in the specific example of acetylene glycolrepresented by the formula (1). As the ethylene oxide adduct of theacetylene glycol represented by the formula (2), commercial product maybe used. For example, Olfine E1004, E1010, and E4300, and Surfynol 485(all the above are merchandise name, manufactured by Nissin ChemicalCo., Ltd.) may be included.

It is preferable that the three or more types of acetylene glycol-basedsurfactants contain one or more types of acetylene glycol represented bythe formula (1) and two or more types of ethylene oxide adducts ofacetylene glycol represented by the formula (2). Thus, the effect by theabove-described three or more types of acetylene glycol-basedsurfactants is further improved.

The total content of the three or more types of acetylene glycol-basedsurfactants is required to be from 0.1 mass % to 3 mass % with respectto the total mass of each of the inks. The total content of thesurfactants is preferably from 0.5 mass % to 2 mass %, and morepreferably from 1 mass % to 2 mass %. The above-described effect isexhibited so as to be further improved, by causing the content of thethree or more types of acetylene glycol-based surfactants to be in theabove range. If the content of the surfactants is less than 0.1 mass o,obtaining of the effect of rapidly infiltrating the ink to a sheet isdifficult.

The content of each of the three or more types of acetylene glycol-basedsurfactants is preferably 5 mass % or more and 50 mass % or less, andpreferably 10 mass % or more and less than 50 mass %, with respect tothe total content of the three or more types of acetylene glycol-basedsurfactants. Mixing balance between the three or more types of acetyleneglycol-based surfactants becomes good by causing the above content to bein the numerical range. Thus, the effect occurring by containing thethree or more types of acetylene glycol-based surfactants is furtherimproved.

In a case where among the three or more types of acetylene glycol-basedsurfactants, a HLB value of a first acetylene glycol-based surfactant isset as h_(l), a HLB value of a second acetylene glycol-based surfactantis set as h₂, and a HLB value of a third acetylene glycol-basedsurfactant is set as h₃, all of the following expressions (A), (B), and(C) are preferably satisfied.

h₁<h₂<h₃   (A)

h₁<6   (B)

10≤h₃   (C)

In this manner, an ink which contains three or more types of acetyleneglycol-based surfactant having a HLB value different from each other isused, and thus the effect of rapidly infiltrating the ink to a sheet isimproved. More specifically, the first acetylene glycol which islipophilic is contained, and thus an effect of wet spreadability to arecording medium having low ink absorptivity is exhibited so as to begood. The third acetylene glycol which is hydrophilic is contained, andthus the effect of wet spreadability to a recording medium having highink absorptivity is exhibited so as to be good. The second acetyleneglycol-based surfactant having a HLB value which is intermediate betweenthe HLB values of the first acetylene glycol-based surfactant and thesecond acetylene glycol-based surfactant is contained, and thus goodcompatibility between the first acetylene glycol-based surfactant andthe third acetylene glycol-based surfactant is provided. Thus, a gooddischarging property in high-speed printing can be obtained. In thismanner, the above-described effect occurring by containing the three ormore types of acetylene glycol-based surfactants is obtained.

The three or more types of acetylene glycol-based surfactants containedin the ink according to the embodiment satisfy the above expressions (A)to (C). In addition, the HLB value (h₂) of the second acetyleneglycol-based surfactant is preferably from 7 to 12 (that is, 7≤h₂≤12),and more preferably from 8 to 12 (that is, 8≤h₂≤12). Thus, the balancebetween the HLB values of the three or more types of acetyleneglycol-based surfactants contained in the ink is good. Accordingly,image quality, fixability, and high-speed recording properties tovarious types of recording mediums may be further improved.

The HLB value (h₁) of the first acetylene glycol-based surfactant ispreferably less than 6 as shown in the expression (B). However, from aviewpoint in that dispersibility of the first acetylene glycol-basedsurfactant in the ink is improved, the HLB value (h₁) is more preferably1 or more and less than 6 (that is, 1≤h₁<6), and further preferably 3 ormore and less than 6 (that is, 3≤h₁<6)).

The HLB value (h₃) of the third acetylene glycol-based surfactant ispreferably equal to or more than 10 as shown in the expression (C).However, from a viewpoint in that good miscibility of the thirdacetylene glycol-based surfactant and other surfactants can be ensuredand infiltration property to a recording medium having high absorptivityof an ink can be further improved, the HLB value (h₃) is more preferablyfrom 10 to 20 (that is, 10≤h₃≤20), and further preferably from 12 to 18(that is, 12≤h₃≤18) ).

In the invention, the HLB value is a value for evaluating hydrophilicityof a compound, which is proposed by Davies and others. For example, theHLB value refers to a numerical value obtained by using Davies methodwhich is defined in the document “J. T. Davies and E. K. Rideal,“Interface Phenomena” 2nd ed. Academic Press, New York 1963” and to avalue calculated by the following expression (i).

HLB value=7+Σ[1]+Σ[2]  (i)

(In the expression (i), [1] indicates a cardinal number of a hydrophilicgroup and [2] indicates a cardinal number of a hydrophobic group)

In a case where, with respect to the total mass (100 mass %) of the ink,the content of the first acetylene glycol-based surfactant is set as W₁mass %, the content of the second acetylene glycol-based surfactant isset as W₂ mass %, and the content of the third acetylene glycol-basedsurfactant is set as W₃ mass %, a ratio (W₁:W₂:W₃) of the above contentsis preferably in a range of 3:1:1 to 1:1:1. Since mixing balance betweenthe three or more types of acetylene glycol-based surfactant becomesgood by causing the ratio to be in the numerical range, the effectoccurring by containing the three or more types of acetyleneglycol-based surfactants is further improved.

It is preferable that the three or more types of acetylene glycol-basedsurfactants contain one or more types of acetylene glycol which is nottransparently dissolved when being added to water so as to have aconcentration of 1%, one or more types of ethylene oxide adducts ofacetylene glycol which is not transparently dissolved when being addedto water so as to have a concentration of 1%, and one or more types ofethylene oxide adducts of acetylene glycol which is transparentlydissolved when being added to water so as to have a concentration of 1%.Thus, the effect by the above-described three or more types of acetyleneglycol-based surfactants may be further improved.

The solubility of the acetylene glycol-based surfactant to water may bedetermined based on light transmittance of a liquid mixture of theacetylene glycol-based surfactant and water. A liquid mixture havinglight transmittance which is equal to or more than 90% is set to “betransparently dissolved”. A liquid mixture having light transmittancewhich is less than 90% is set to “be not transparently dissolved”. Theacetylene glycol-based surfactant is added to water so as to have aconcentration of 1%, and mixing and stirring is performed for 10 minutesor longer, thereby the liquid mixture is prepared. The lighttransmittance may be measured by using a spectrophotometer and by usinga quartz cell having an optical path length of 1 cm. An example of thespectrophotometer used in the measurement includes U-3900 (merchandisename) manufactured by Hitachi High-Technologies Corporation.

1.3. Alkylene Glycol Monoalkyl Ether

The ink constituting the ink set according to the embodiment may containalkylene glycol monoalkyl ether. In a case where the ink constitutingthe ink set according to the embodiment contains alkylene glycolmonoalkyl ether, alkylene glycol monoalkyl ether functions as aninfiltrating agent, and thus an ink is infiltrated to a sheet rapidly,and the effect of suppressing the occurrence of transfer of an ink tothe sheet transporting mechanism is obtained.

Examples of alkylene glycol monoalkyl ether include ethylene glycolmonoisobutyl ether, ethylene glycol monohexyl ether, ethylene glycolmonoisohexyl ether, diethylene glycol monohexyl ether, triethyleneglycol monobutyl ether (TEGmBE), diethylene glycol monoisohexyl ether,triethylene glycol monoisohexyl ether, ethylene glycol mono isoheptylether, diethylene glycol monoisoheptyl ether, triethylene glycolmonoisoheptyl ether, ethylene glycol monooctyl ether, ethylene glycolmonoisooctyl ether, diethylene glycol monoisooctyl ether, triethyleneglycol monoisooctyl ether, ethylene glycol mono-2-ethylhexyl ether,diethylene glycol mono-2-ethylhexyl ether, triethylene glycolmono-2-ethylhexyl ether, diethylene glycol mono-2-ethyl pentyl ether,ethylene glycol mono-2-ethyl pentyl ether, ethylene glycolmono-2-ethylhexyl ether, diethylene glycol mono-2-ethylhexyl ether,ethylene glycol mono-2-methyl pentyl ether, diethylene glycolmono-2-methyl pentyl ether, propylene glycol monobutyl ether,dipropylene glycol monobutyl ether, tripropylene glycol monobutyl ether,propylene glycol monopropyl ether, dipropylene glycol monopropyl ether,and tripropylene glycol monomethyl ether.

The alkylene glycol monoalkyl ethers may be singly used or two or morethereof may be mixed and used.

In a case where the alkylene glycol monoalkyl ether is contained, thecontent may be set to be from 0.5 mass % to 6 mass % with respect to thetotal mass of the ink.

1.4. 1,2-alkane diol

The ink constituting the ink set according to the embodiment may contain1,2-alkane diol. In a case where the ink constituting the ink setaccording to the embodiment contains 1,2-alkane diol, 1,2-alkane diolfunctions as an infiltrating agent, and thus an ink is infiltrated to asheet rapidly, and the effect of suppressing the occurrence of transferof an ink to the sheet transporting mechanism is obtained.

Examples of 1,2-alkane diols include 1,2-propanediol, 1,2-butanediol,1,2-pentanediol, 1,2-hexanediol, and 1,2-octanediol.

1,2-alkane diols may be singly used or two or more thereof may be mixedand used.

In a case where the 1,2-alkane diols is contained, the content may beset to be from 1 mass % to 20 mass % with respect to the total mass ofthe ink.

1.5. Trimethylglycine

The ink constituting the ink set according to the embodiment may containtrimethylglycine (also referred to as glycinebetaine). Trimethylglycineis one of betaine compounds which function as a moisturizing agent.Trimethylglycine is a substance which is excellent in curling andcockling aptitudes, and is excellent in viscosity characteristicsdepending on the temperature. Thus, in a case where the ink constitutingthe ink set according to the embodiment contains trimethylglycine, it ispossible to suppress the occurrence of permanent curling on a sheet,improve the sheet transporting properties, and to allow speeding-up ofthe duplex printing.

In a case where the trimethylglycine is contained, the content thereofmay be set to be from 1 mass % to 40 mass %, and to be preferably from 5mass % to 30 mass %, with respect to the total mass of the ink. As acommercial product of trimethylglycine, for example, AMINOCOAT(manufactured by Asahi Kasei Chemical Corporation) may be used.

1.6. Other Components

The ink constituting the ink set according to the embodiment may containother organic solvents, other resins, a surfactant (also referred to as“other surfactant” below) other than the acetylene glycol-basedsurfactant having the above-described specific structure, a pH adjuster,a corrosion inhibitor or antifungal agent, a rust inhibitor, a chelatingagent, and the like.

1.6.1. Organic Solvent

The organic solvent is not particularly limited. Examples of the organicsolvent include polyhydric alcohols and pyrrolidone derivatives.

Examples of polyhydric alcohols include ethylene glycol, diethyleneglycol, propylene glycol, dipropylene glycol, 1,3-propanediol,1,4-butanediol, 1,6-hexanediol, trimethylol propane, and glycerine.Polyhydric alcohols include a function of improving wettability of anink to a sheet, a function of improving a moisture-retaining propertyfor a nozzle of a recording head, or the like. In a case where thepolyhydric alcohols are contained, the content may be set to be from 2mass % to 30 mass o, with respect to the total mass of the ink.

Examples of the pyrrolidone derivatives include N-methyl-2-pyrrolidone,N-ethyl-2-pyrrolidone, N-vinyl-2-pyrrolidone, 2-pyrrolidone,N-butyl-2-pyrrolidone, and 5-methyl-2-pyrrolidone. The pyrrolidonederivatives may function as a good solubilizer for a resin. In a casewhere the pyrrolidone derivatives are contained, the content may be setto be from 0.5 mass % to 6 mass %, with respect to the total mass of theink.

1.6.2. Resin

The resin includes a function of improving fixability of the ink.Examples of the resin include acrylic-based resins, fluorene-basedresins, urethane-based resins, olefin-based resins, rosin-modifiedresins, terpene-based resins, ester-based resins, amide-based resins,epoxy-based resins, vinyl chloride-based resins, vinyl chloride-vinylacetate copolymer, and ethylene-vinyl acetate-based resins. Theabove-described resins may be singly used or be used in combination oftwo or more types. As the resin, an emulsion type resin may be used or asolution type resin may be used. In a case where the resin is contained,the content thereof may be set to be from 0.1 mass % to 6 mass %.

1.6.3. Other Surfactants

The ink constituting the ink set according to the embodiment may containother surfactant (surfactants other than the acetylene glycol-basedsurfactant having the above-described specific structure). Examples ofsuch surfactants may include acetylene glycol-based surfactants which donot correspond to the above formulas (1) and (2), silicone-basedsurfactants, and fluorine-based surfactants.

1.6.4. pH Adjuster, Corrosion Inhibitor or Antifungal Agent, RustInhibitor, and Chelating Agent

Examples of the pH adjuster include Good's buffers, phosphate buffersolutions, and TRIS buffer solutions such as potassium dihydrogenphosphate, disodium hydrogen phosphate, sodium hydroxide, lithiumhydroxide, potassium hydroxide, ammonia, diethanolamine,triethanolamine, triisopropanolamine, potassium carbonate, sodiumcarbonate, sodium hydrogen carbonate, tris(hydroxymethyl)amino methane(THAM), 4-(2-hydroxyethyl)-1-piperazine ethane sulfonic acid (HEPES),morpholino ethanesulfonic acid (MES), carbamoylmethyl imino bis aceticacid (ADA), piperazine-1,4-bis(2-ethane sulfonic acid) (PIPES),N-(2-acetamide)-2-amino ethanesulfonic acid (ACES), colaminehydrochloride, N, N-bis(2-hydroxyethyl)-2-aminoethane sulfonic acid(BES), N-tris (hydroxymethyl)methyl-2-amino ethanesulfonic acid (TES),acetamide glycine, tricine, glycine amide, and bicine.

Examples of the corrosion inhibitor and the antifungal agent includesodium benzoate, pentachlorophenol sodium, 2-pyridinethiol-1-oxidesodium, sodium sorbate, sodium dehydroacetate, 1,2-dibenzoisothiazolin-3-on (PROXEL CRL, PROXEL BDN, PROXEL GXL, PROXEL XL.2,PROXEL TN, and PROXEL LV of Zeneca Corporation), and4-chloro-3-methylphenol (PREVENTOL CMK of Bayer Holding Ltd.).

An example of the rust inhibitor includes benzotriazole.

Examples of the chelating agent include ethylenediamine tetraaceticacids and salts thereof (ethylenediamine tetraacetic acid disodiumdihydrogen salt and the like).

1.7. Preparing Method of Ink

The ink constituting the ink set according to the embodiment is obtainedin such a manner that the above-described components are mixed to eachother in certain order, and if necessary, impurities are removed byfiltering and the like. As a mixing method of the components, a methodin which materials are sequentially put into a container which includesa stirring device such as a mechanical stirrer and a magnetic stirrer,and stirring and mixing is performed is appropriately used. As afiltering method, centrifugal filtration, filter filtration, and thelike may be performed as necessary. 1.8. Physical properties

Regarding the ink constituting the ink set according to the embodiment,from a viewpoint of balance between image quality and reliability as anink for ink jet recording, surface tension at 20° C. is preferably from20 mN/m to 40 mN/m, and more preferably from 25 mN/m to 35 mN/m. Thesurface tension may be measured by using, for example, an automaticsurface tension measuring device CBVP-Z (merchandise name, manufacturedby Kyowa Interface Science Co., Ltd.), and may be measured by confirmingsurface tension when a platinum plate is wet with the ink under anenvironment of 20° C.

In order to allow high-speed infiltration with low viscosity, viscosityof the ink at 20° C. is preferably from 2.0 mm²/s to 4.0 mm²/s, and morepreferably from 2.8 mm²/s to 3.2 mm²/s. The viscosity may be measuredunder an environment of 20° C. by using, for example, a viscoelasticitytester MCR-300 (merchandise name, manufactured by Pysica Corporation).

As described above, according to the embodiment, the moisture quantityof the ink constituting an ink set is adjusted, and thus it is possibleto provide an ink set which can suppress the occurrence of transfer ofthe ink and deformation of a sheet, and can cause a speed of duplexprinting to be increased.

2. DUPLEX PRINTING METHOD

A duplex printing method according to an embodiment of the invention isperformed as follows. The above-described ink set is mounted in an inkjet recording apparatus which includes sheet transporting mechanism(which will be described later) in which a period from when printing ona front surface is started, until printing on a back surface is startedis within three seconds. Ink droplets of an ink constituting the ink setare ejected so as to adhere the ink droplets to a recording medium.Thus, both surfaces of a sheet which corresponds to the recording mediumare printed. In the duplex printing method according to the embodiment,above-described ink set is used. Thus, it is possible to suppress theoccurrence of transfer of the ink to the sheet transporting mechanism,to improve the sheet transporting properties, and to allow speeding-upof the duplex printing.

2.1. Configuration of Apparatus

As the ink jet recording apparatus used in the duplex printing methodaccording to the embodiment, for example, a well-known ink jet printerand the like is used. As such an ink jet printer, any of a serial typeink jet recording apparatus in which a serial head is employed as arecording head, and a line type ink jet recording apparatus in which aline head is employed as a recording head may be used. Here, detaileddescriptions will be made by using the line type ink jet recordingapparatus which employs a line head, as an example.

FIGURE illustrates a side sectional structure of an ink jet recordingapparatus used in the embodiment. The ink jet recording apparatusincludes a main body frame 10 which has a quadrangular box. In thefollowing descriptions, in the same drawing, the right side is set asthe front side of the ink jet recording apparatus, and the left side isset as the rear side of the ink jet recording apparatus.

An operation panel 11 which functions as a user interface is provided ona front side surface of the main body frame 10. An operation switch or aliquid crystal panel is provided in the operation panel 11. Anextendable ejection tray 25 in which a sheet P to be printed is mountedis provided at the front portion of the main body frame 10. An openablemanual feeding port cover 12 is provided on the rear upper side of themain body frame 10. A manual feeding port 22 for manually inserting thesheet P is opened by closing the manual feeding port cover 12.

A sheet tray 13 is provided at a lower portion of the main body frame10, so as to be attachable. In the sheet tray 13, plural pieces ofsheets P may be accommodated in a state of being stacked. In the mainbody frame 10, a feeding mechanism 34 for feeding the sheet P in thesheet tray 13 is provided over the sheet tray 13. The feeding mechanism34 includes a rocking arm 14 and a pickup roller 15. The rocking arm 14is journaled to the main body frame 10 so as to be swingable. The pickuproller 15 is provided at a tip end portion of the rocking arm 14. Thepickup roller 15 delivers the top sheet of the stack of the sheets Paccommodated in the sheet tray 13 rearwardly in accordance with therotation of the pickup roller 15. The wall surface on the rear of thesheet tray 13 functions as a separation inclined-surface 16 which isinclined rearwardly with being upward.

In the ink jet recording apparatus, a transporting path C1 of a sheet Pfed by the sheet tray 13 is formed so as to be turned over forward andbe directed to the ejection tray 25 after being directed to theseparation inclined-surface 16 from the stack of the sheets P in thesheet tray 13, and being directed upwardly by the separationinclined-surface 16. A transporting path C2 of a sheet P fed by themanual feeding port 22 is joined at a portion of the transporting pathC1 just after the sheet P is turned over, from the manual feeding port22, and is common with the transporting path C1. In the followingdescriptions, feeding sides in the transporting paths C1 and C2 arereferred to an upstream of the transporting paths C1 and C2, andejecting sides are referred to a downstream of the transporting paths C1and C2.

In the transporting path C1, an intermediate roller 17 is provided as anexample of a second transporting roller, on a downstream of theseparation inclined-surface 16. A separation roller 18 is provided onthe intermediate roller 17 so as to be adjacent to the intermediateroller 17. The separation roller 18 is rotated by the rotation of theintermediate roller 17. An outer circumferential surface of theseparation roller 18 is formed by an elastic material and is pressed onthe intermediate roller 17 with a defined force. Thus, appropriaterotation resistance is applied to a sheet P which passes between theintermediate roller 17 and the separation roller 18, and thus feeding ofoverlapped sheets P is prevented.

An intermediate assist roller 19 is provided on a downstream of theseparation roller 18 in the transporting path Cl. The intermediateassist roller 19 is provided in a state of being pressed on theintermediate roller 17, and is rotated by the rotation of theintermediate roller 17.

In the transporting paths C1 and C2, a sheet feeding roller 20 isprovided as an example of a first transporting roller, on a downstreamof the intermediate assist roller 19. A sheet delivery assist roller 21is pressed on the sheet feeding roller 20. The sheet delivery assistroller 21 is rotated by the rotation of the sheet feeding roller 20. Asheet detecting sensor is provided in the vicinity of the sheet feedingroller 20 on the upstream side of the transporting paths C1 and C2. Thesheet detecting sensor detects whether or not a sheet P is provided atthe position of the sheet detecting sensor. As the sheet detectingsensor 40, for example, an optical sensor is used.

An ejecting roller 23 is provided on a downstream of the sheet feedingroller 20 in the transporting paths C1 and C2. An ejecting assist roller24 is pressed on the ejecting roller 23. The ejecting assist roller 24is rotated by the rotation of the ejecting roller 23.

A head unit 27 is provided in the main body frame 10. The head unit 27is supported by a support section 26. An ink jet type recording head 28is provided at a lower portion of the head unit 27. The recording head28 is a line type ink jet head including a long nozzle row which isprovided over the entirety of the width of the sheet P in a widthdirection thereof. In a case where color printing is performed, a nozzlerow of each color for discharging ink droplets of the correspondingcolor is formed on a nozzle surface of the line type ink jet head. Inaddition, a configuration in which a line type ink jet head of eachcolor which includes a nozzle row of the corresponding color fordischarging ink droplets of the corresponding color is included may bemade. Here, the line type ink jet head is used with the meaning ofincluding all of the above-described cases. When printing is performed,the recording head 28 is set to be positioned over a section between thesheet feeding roller 20 and the ejecting roller 23 in the transportingpaths C1 and C2.

A support base 29 which supports a sheet P is provided at a position inthe main body frame 10, which faces the recording head 28. The head unit27 is provided in a vertical direction in the drawing, so as to enablegoing up and down. A gap between the recording head 28 and the supportbase 29 may be adjusted in accordance with the ascent and the descent(described as gap adjustment below).

The ink jet recording apparatus includes a motor (not illustrated) as adriving source. In the ink jet recording apparatus, the motor rotatesthe sheet feeding roller 20 and the ejecting roller 23. Rotationdirections of the sheet feeding roller 20 and the ejecting roller 23 areswitched in accordance with a rotation direction of the motor. Aconfiguration in which the power of the motor 31 is also transferred tothe intermediate roller 17 and the rotation direction of theintermediate roller 17 is maintained so as to be constant is made.

A controller (not illustrated) which performs various types of controlfor the ink jet recording apparatus is provided. The controller isconnected to the operation panel 11. The controller confirms anoperation status of an operation switch provided in the operation panel11, and controls a display of a liquid crystal panel which is providedin the same operation panel 11.

In the embodiment, the intermediate roller 17, the separation roller 18,the intermediate assist roller 19, the sheet feeding roller 20, thesheet delivery assist roller 21, the ejecting roller 23, the ejectingassist roller 24, and the like constitute the sheet transportingmechanism. In the embodiment, the sheet transporting mechanism is set tocause a period from when printing on the front surface of a sheet P isstarted until printing on the back surface is started to be within threeseconds. That is, in a case where plain paper is used as a sheet P, aduplex printing speed (pages/minute) in printing a solid image on theentirety of a surface of the sheet P is set to be from 30% of a simplexprinting speed, to 100% thereof.

2.2. Duplex Printing

Details of the duplex printing method according to the embodiment willbe described. The controller determines printing conditions inaccordance with the operation status of the operation switch provided inthe operation panel 11. For example, it is determined whether manualfeeding is performed or tray feeding is performed, based on informationof a feeding type, which is included in printing condition information.The size of a gap between the recording head and the support base 29during printing is determined based on information of the type of asheet, printing quality, and the like, which is included in the printingcondition information.

The controller confirms the state of the recording head 28 and performsgap adjustment. Here, in a case where tray feeding is performed, afterthe controller completes the gap adjustment, the controller controls themotor to be normally rotated, and controls the pickup roller 15 to berotated. Thus, a sheet P is delivered from the stack in the sheet tray13. The delivered sheet P is transported on the transporting path C1 bythe intermediate roller 17 and the sheet feeding roller 20, until theleading end of the sheet P reaches a printing start position.

In a case where manual feeding is performed, after the controllercompletes the gap adjustment, the controller waits until a user operatesa feeding start button provided in the operation panel 11. If thefeeding start button is operated, the controller controls the motor tobe normally rotated. Thus, a sheet P is transported by the intermediateroller 17 and the sheet feeding roller 20, until the leading end of thesheet P reaches a printing start position. If the leading end of thesheet P reaches a printing start position, the controller startsprinting on the front surface.

After printing on the front surface of the sheet P is completed, thecontroller performs a reversing operation of a print surface of thesheet P. The reversing operation is performed by causing the motor to bereversely rotated. If the motor is reversely rotated, the sheet feedingroller is reversely rotated, and thus the sheet P having the frontsurface printed thereon is transported to the rear side of the printingapparatus. The delivered sheet P is firstly inserted into a lower sideof the intermediate roller 17. The sheet P turns around the intermediateroller 17 by the intermediate roller 17 which is normally rotated. Then,the sheet P is caused to return to the sheet feeding roller 20 side froma upper portion side of the same intermediate roller 17. Thus, the printsurface of the sheet P is reversed from the front surface to the backsurface. After the entirety of the sheet P passes through the sheetfeeding roller 20, the controller reverses the rotation direction of themotor. The controller starts printing on the back surface in a statewhere the sheet P is transported until the leading end thereof reachesthe printing start position. If printing on the back surface iscompleted, the controller ejects the sheet P on which printing iscompleted, to the ejection tray 25, and ends a printing operation.

As described above, according to the embodiment, the moisture quantityof the ink constituting an ink set is adjusted, and thus the occurrenceof deformation of a sheet is suppressed. Since the ink is infiltrated toa sheet and dried during a period from when printing on the frontsurface is started until printing on the back surface is started, theoccurrence of transfer of the ink is suppressed. Accordingly, it ispossible to increase the speed of duplex printing.

3. EXAMPLE

The invention will be specifically described below by using experimentalexamples and comparative examples. However, the invention is not limitedto only the examples.

3.1. Preparation of Ink

An ink was prepared in the manner disclosed in JP-A-2011-178916, byusing materials shown in Table 1. As shown in Table 2, in the example,the moisture quantity of each ink was adjusted so as to be from 57 mass% to 84 mass %. In the example, four inks (self-dispersing black ink,self-dispersing cyan ink, self-dispersing magenta ink, andresin-dispersion yellow ink) were manufactured only by changing the typeof a pigment, and each of the four inks was independently evaluated.

TABLE 1 Composition of ink Added amount [mass %] Pigment 6 InfiltratingAgent TEGmBE 2 Infiltrating Agent 1,2-hexanediol 2 Moisturizing AgentGlycerine Adjustment Moisturizing Agent Trimethylglycine 5 Surfactant 1Surfynol 104 0.4 Surfactant 2 Olfine E1004 0.4 Surfactant 3 Olfine E10100.4 Moisture Quantity Adjustment Total 100 TEGmBE (triethylene glycolmonobutyl ether)

In Table 1, materials having a description other than the compound nameare as follows.

-   Pigment (S170 (for self-dispersing black ink), C.I. Pigment Blue    15:4 (for self-dispersing cyan ink), C.I. Pigment Red 122 (for    self-dispersing magenta ink), or C.I. Pigment Yellow 74 (for    resin-dispersion yellow ink) was used for each of the inks)-   Surfynol 104 (merchandise name, manufactured by Nissin Chemical Co.,    Ltd., acetylene glycol corresponding to the formula (1),    2,4,7,9-tetramethyl-5-decyne-4,7-diol)-   Olfine E1004 (merchandise name, manufactured by Nissin Chemical Co.,    Ltd., ethylene oxide adduct of acetylene glycol corresponding to the    formula (2))-   Olfine E1010 (merchandise name, manufactured by Nissin Chemical Co.,    Ltd., ethylene oxide adduct of acetylene glycol corresponding to the    formula (2), and ethylene oxide adduct of    2,4,7,9-tetramethyl-5-decyne-4,7-diol (the number of moles of added    ethylene oxide: 10))

3.2. Evaluation Test

The following evaluation test was performed by using a sample which wasobtained in the following manner. Firstly, a recording head of an inkjet printer PX-M7050F (manufactured by Seiko Epson Corporation) wasmodified to be a line head. Modification was performed so as to change aspeed in a sheet transporting path, and a test for an interval betweensimplex printing and duplex printing was performed. Then, an inkcartridge filled with the inks prepared as described above was mountedin the printer and each of the inks was discharged from nozzles of therecording head. Thus, an image was recorded on a recording medium.

3.2.1. Status of Sheet Transporting

The N6A.Tif pattern of J color digital reference image data ISO/JIS-SCIDwas printed on both surfaces of A4 Business plain paper (manufactured bySeiko Epson Corporation). The printing was performed on the total 200pages of the total 100 pieces of paper, and it was determined whether ornot paper jam occurred. Evaluation criteria are as follows.

-   A: no paper jam during one second from when printing on the front    surface is started until printing on the back surface is started-   B: no paper jam during three seconds from when printing on the front    surface is started until printing on the back surface is started-   C: paper jam occurs during three seconds from when printing on the    front surface is started until printing on the back surface is    started

3.2.2. Transfer of Ink to Duplex Transporting Roller

The N6A.Tif pattern of color digital reference image data ISO/JIS-SCIDwas printed on both surfaces of A4 Business plain paper (manufactured bySeiko Epson Corporation). The printing was performed on the total 200pages of the total 100 pieces of paper, and it was determined whether ornot a trace on the duplex transporting roller which performed transferof an ink adhered to the paper. Evaluation criteria are as follows.

-   A: adhering of a trace of the duplex transporting roller is not    confirmed-   B: adhering of a trace of the duplex transporting roller is    confirmed slightly.-   C: adhering of a trace of the duplex transporting roller is    confirmed

3.2.3. Presence or Absence of Permanent Curling

The N6A.Tif pattern of J color digital reference image data ISO/JIS-SCIDwas printed on the entirety of one surface of A4 Business plain paper(manufactured by Seiko Epson Corporation). The paper was left at 24° C.at humidity of 40% for 24 hours, and then it was determined whether ornot curling occurred in four edges of the A4 paper. Evaluation criteriaare as follows.

-   A: the height of the curling is less than 2 cm-   B: the height of the curling is equal to or more than 2 cm

3.3. Evaluation Results

Table 2 shows results of the above-described evaluations. Regarding theself-dispersing black ink and the resin-dispersion yellow ink, in a casewhere the moisture quantities thereof were equal to or less than 73 mass%, any ink did not cause the occurrence of paper jam during one secondfrom when printing on the front surface was started until printing onthe back surface was started. When the moisture quantities thereof were74 mass % and 75 mass %, any ink did not cause the occurrence of paperjam during three seconds from when printing on the front surface wasstarted until printing on the back surface was started. On the contrary,when the moisture quantities thereof were equal to or more than 76 mass%, the paper jam occurred during three seconds from when printing on thefront surface was started until printing on the back surface wasstarted. Regarding the transfer of an ink to the duplex transportingroller, when the moisture quantities thereof were equal to or less than67 mass %, the transfer of an ink was confirmed. However, when themoisture quantities thereof were 68 mass % and 69 mass %, the transferof an ink was slightly confirmed. When the moisture quantities thereofwere equal to or more than 70 mass %, the transfer of an ink was notconfirmed. Regarding the presence or the absence of the permanentcurling, the self-dispersing black ink did not cause the permanentcurling to occur when the moisture quantity thereof was equal to or lessthan 72 mass %. The resin-dispersion yellow ink did not cause thepermanent curling to occur when the moisture quantity thereof was equalto or less than 73 mass %. Thus, regarding the self-dispersing black inkand the resin-dispersion yellow ink, in a case where the moisturequantities thereof were from 68 mass % to mass %, it was understood thatthe sheet transporting properties were good, and the occurrence of thetransfer of an ink to the duplex transporting roller was suppressed.

Regarding the self-dispersing cyan ink and the self-dispersing magentaink, in a case where the moisture quantities thereof were equal to orless than 70 mass %, any ink did not cause the occurrence of paper jamduring one second from when printing on the front surface was starteduntil printing on the back surface was started. When the moisturequantities thereof were 71 mass % and 72 mass %, any ink did not causethe occurrence of paper jam during three seconds from when printing onthe front surface was started until printing on the back surface wasstarted. On the contrary, when the moisture quantities thereof wereequal to or more than 73 mass %, the paper jam occurred during threeseconds from when printing on the front surface was started untilprinting on the back surface was started. Regarding the transfer of anink to the duplex transporting roller, when the moisture quantitiesthereof were equal to or less than 61 mass %, the transfer of an ink wasconfirmed. However, when the moisture quantities thereof were from 62mass % to 65 mass %, the transfer of an ink was slightly confirmed. Whenthe moisture quantities thereof were equal to or more than 66 mass %,the transfer of an ink was not confirmed. Regarding the presence or theabsence of the permanent curling, when the moisture quantity thereofwere equal to or less than 74 mass %, the permanent curling did notoccur. Thus, regarding the self-dispersing cyan ink and theself-dispersing magenta ink, in a case where the moisture quantitiesthereof were from 62 mass % to 72 mass %, it was understood that thesheet transporting properties were good, and the occurrence of thetransfer of an ink to the duplex transporting roller was suppressed.

TABLE 2 Moisture quantity [mass %] 57 58 59 60 61 62 63 64 65 66 67 6869 70 Self- Status of sheet A A A A A A A A A A A A A A dispersingtransporting black Transfer of ink to C C C C C C C C C C C B B A duplextransporting roller Permanent curling B B B B B B B B B B B B B B Self-Status of sheet A A A A A A A A A A A A A A dispersing transporting cyanTransfer of ink to C C C C C B B B B A A A A A duplex transportingroller Permanent curling B B B B B B B B B B B B B B Self- Status ofsheet A A A A A A A A A A A A A A dispersing transporting magentaTransfer of ink to C C C C C B B B B A A A A A duplex transportingroller Permanent curling B B B B B B B B B B B B B B Resin- Status ofsheet A A A A A A A A A A A A A A dispersion transporting yellowTransfer of ink to C C C C C C C C C C C B B A duplex transportingroller Permanent curling B B B B B B B B B B B B B B Moisture quantity[mass %] 71 72 73 74 75 76 77 78 79 80 81 82 83 84 Self- Status of sheetA A A B B C C C C C C C C C dispersing transporting black Transfer ofink to A A A A A A A A A A A A A A duplex transporting roller Permanentcurling B B C C C C C C C C C C C C Self- Status of sheet B B C C C C CC C C C C C C dispersing transporting cyan Transfer of ink to A A A A AA A A A A A A A A duplex transporting roller Permanent curling B B B B CC C C C C C C C C Self- Status of sheet B B C C C C C C C C C C C Cdispersing transporting magenta Transfer of ink to A A A A A A A A A A AA A A duplex transporting roller Permanent curling B B B B C C C C C C CC C C Resin- Status of sheet A A A B B C C C C C C C C C dispersiontransporting yellow Transfer of ink to A A A A A A A A A A A A A Aduplex transporting roller Permanent curling B B B C C C C C C C C C C C

3.4. Examination of Speeding-Up of Duplex Printing

Next, speeding-up of duplex printing was examined. In a case whereduplex printing was performed by using an ink of four colors, in a statewhere an interval from when printing on the front surface is starteduntil printing on the back surface is started was set to be one secondto ten seconds, it was examined whether or not paper jam occurred. TheN6A.Tif pattern of J color digital reference image data ISO/JIS-SCID wasprinted on both surfaces of the following three types of recordingmediums. The printing was performed on the total 200 pages of the total100 pieces of paper, and it was determined whether or not paper jamoccurred. Evaluation criteria are as follows.

-   A: no paper jam-   B: paper jam occurs

The used recording media are as follows.

-   Plain paper A (merchandise name “plain paper with both surfaces of    high quality”, manufactured by Seiko Epson Corporation)-   Plain paper B (merchandise name “CopyPlusPaper”, manufactured by    Hammermill Corporation)-   Fine coating paper for printing (merchandise name “OK Topcoat+”,    manufactured by Oji Paper Co., Ltd.)

Regarding Example 1 and Comparative Example 1, in a case where duplexprinting was performed at a duplex printing speed (pages/minute) of theplain paper A which was set to be from 0% of a simplex printing speed to100% thereof, it was examined whether or not paper jam occurred.

3.5. Evaluation Results

Tables 3 and 4 show results of the above-described evaluations. As shownin Table 3, in Examples 1 and 2 in which the moisture quantity of theink of four colors was 70 mass %, no paper jam occurred even when theinterval from when printing on the front surface was started untilprinting on the back surface was started was one second, and it wasunderstood that the speeding-up of the duplex printing was possible. Onthe contrary, in Comparative Example 1 in which the moisture quantity ofthe ink of four colors was more than the range in the invention, and inComparative Example 2 in which the moisture quantity of the ink of fourcolors was less than the range in the invention, the paper jam did notoccur when the interval from when printing on the front surface wasstarted until printing on the back surface was started was equal to orlonger than four seconds, but the paper jam occurred when the intervalwas equal to or shorter than three seconds. If the above results werecompared to the results of Examples 1 and 2, the speeding-up of theduplex printing was not possible. As described as the experimentalexample, in a case where fine coating paper for printing was used as arecording medium, an ink was not infiltrated to the recording medium.Thus, even when the interval from when printing on the front surface wasstarted until printing on the back surface was started was ten seconds,the paper jam occurred.

Table 4 shows results indicated by a ratio of the duplex printing speedto the simplex printing speed. In Example 1 in which the moisturequantity of the ink of four colors was 70 mass %, no paper jam occurred,and even when the duplex printing speed did not become slow, no paperjam occurred. However, in Comparative Example 1 in which the moisturequantity of the ink of four colors was more than the range in theinvention, if the duplex printing speed was set not to be reduced to beequal to or less than 20% of the simplex printing speed, the paper jamoccurred.

In this manner, in a case where the moisture quantity of the ink isincluded in the range in the invention, it was understood that thespeeding-up of the duplex printing was possible even in full-colorprinting in which the amount of adhered ink is large. In this manner, itwas understood that high-speed duplex printing as fast as that of alaser printer was possible even when the moisture quantity is definedbased on the type of a pigment, setting as a composition according tothe invention was performed, and thus a line type ink jet printer wasused.

TABLE 3 ← Duplex printing speeding-up Interval between simplex printingand duplex printing [second] 1 2 3 4 5 6 7 8 9 10 Example 1 A A A A A AA A A A Moisture quantity of ink of four colors, which is constantly 70%Plain paper A Example 2 A A A A A A A A A A Moisture quantity of ink offour colors, which is constantly 70% Plain paper B Comparative Example 1B B B A A A A A A A Moisture quantity of ink of four colors, which isconstantly 74% Plain paper A Comparative Example 2 B B B A A A A A A AMoisture quantity of ink of four colors, which is constantly 66% Plainpaper A Experimental Example B B B B B B B B B B Moisture quantity ofink of four colors, which is constantly 70% Fine coating paper forprinting

TABLE 4 Speed [%] in duplex printing (in comparison to speed in simplexprinting 100 90 80 70 60 50 40 30 20 10 0 Example 1 A A A A A A A A A AA Moisture quantity of ink of four colors, which is constantly 70% Plainpaper A Comparative B B B B B B B B A A A Example 1 Moisture quantity ofink of four colors, which is constantly 74% Plain paper A

As described above, according to the embodiment, the moisture quantityof the ink constituting an ink set is adjusted, and thus it is possibleto provide an ink set which can suppress the occurrence of transfer ofthe ink and deformation of a sheet, and can cause a speed of duplexprinting to be increased.

The invention is not limited to the above-described embodiment, andvarious modifications may be applied. For example, the inventionincludes a configuration (for example, configuration having the samefunction, the same method, and the same results, or configuration havingthe same object and the same effects) which is substantially the same asthe configuration described in the embodiment. The invention includes aconfiguration obtained by substituting a component which is notfundamental component in the configuration described in the embodiment.The invention includes a configuration which can exhibit the sameadvantages effects the same as those of the configuration described inthe embodiment, or can achieve the same object. The invention includes aconfiguration obtained by adding well-known technologies to theconfiguration described in the embodiment.

What is claimed is:
 1. A duplex printing method of performing duplexprinting by using an ink jet recording apparatus, wherein the ink jetrecording apparatus includes an ink set and a sheet transportingmechanism, the ink set contains at least one selected from a group of ablack pigment ink, a self-dispersing color pigment ink, and aresin-dispersion color pigment ink, a moisture quantity of the blackpigment ink is from 68 mass % to 75 mass %, a moisture quantity of theself-dispersing color pigment ink is from 62 mass % to 72 mass %, amoisture quantity of the resin-dispersion color pigment ink is from 68mass % to 75 mass %, and in the sheet transporting mechanism, a periodfrom when printing on a front surface is started, until printing on aback surface is started is within three seconds.
 2. The duplex printingmethod according to claim 1, wherein a duplex printing speed(page/minute) in printing a solid image on the entirety of a surface ofplain paper is from 30% to 100% of a simplex printing speed.
 3. Theduplex printing method according to claim 1, wherein the black pigmentink is formed from a self-dispersing pigment.
 4. The duplex printingmethod according to claim 1, wherein each of the black pigment ink, theself-dispersing color pigment ink, and the resin-dispersion colorpigment ink contains three or more types of acetylene glycol-basedsurfactants selected from a group of acetylene glycol and an ethyleneoxide adduct of acetylene glycol, and the total content of the acetyleneglycol-based surfactants is from 0.1 mass % to 3 mass % with respect tothe total mass of each of the black pigment ink, the self-dispersingcolor pigment ink, and the resin-dispersion color pigment ink.
 5. Theduplex printing method according to claim 1, wherein the viscosity ofeach of the black pigment ink, the self-dispersing color pigment ink,and the resin-dispersion color pigment ink is from 2.0 mm²/s to 4.0mm²/s.
 6. The duplex printing method according to claim 1, wherein eachof the black pigment ink, the self-dispersing color pigment ink, and theresin-dispersion color pigment ink further contains alkylene glycolmonoalkyl ether.
 7. The duplex printing method according to claim 1,wherein each of the black pigment ink, the self-dispersing color pigmentink, and the resin-dispersion color pigment ink further contains1,2-alkane diol.
 8. The duplex printing method according to claim 1,wherein each of the black pigment ink, the self-dispersing color pigmentink, and the resin-dispersion color pigment ink further containstrimethylglycine.